Safety switches are known to be used for monitoring personnel access to an area delimited by protection barriers, and containing hazardous materials, mechanical moving parts and/or electric power devices.
Particularly, hinged safety switches have a casing designed to be secured to the fixed part of the protection barrier and a movable member designed to be connected to the door of the barrier.
The casing houses an electric contact unit connected to one or more electric safety circuits and adapted to cause one or more electric contacts to open and/or close upon rotation of the movable part relative to the fixed enclosure.
This configuration affords the movable part of the barrier to be opened or closed by detection of the electric quantities of the signals in the safety circuits.
A need is particularly felt in the art for hinged safety switches that can generate redundant electric signals as a function of whether the barrier is opened and/or closed.
The generation of such redundant signals increases installation flexibility of the switch and provides improvements in switch safety.
This is because the generation of redundant signals may prevent wrong barrier-open and/or barrier-closed signaling, caused by flickering of the contacts of the electric unit of by transient electric arcs formed therebetween or by malfunctioning thereof.
Furthermore, the generation of redundant signals associated with barrier opening and/or closing is often required by standards that regulate the field of electric plants and safety.
Hinged electric devices that accommodate two distinct electric contact units therein have been developed to manufacture this kind of safety switches.
GB2150757 discloses a hinged switch comprising a through central pin about which the movable part pivots, and a pair of electric contact units, disposed within the casing. The electric units have a first portion integral with the casing and a second portion adapted to be connected to the external safety circuit, and attached to the movable part of the switch via two pairs of screws.
As the barrier is opened and/or closed, the portions of each unit are caused to mutually pivot about the pin, with the electric contacts being consequently switched.
Thus, as the barrier is opened and/or closed, two redundant electric signals are generated in the electric safety circuits associated with the contact units.
While such hinged device generates redundant electric signals as a function of whether the barrier is opened or closed, it still suffers from a few recognized drawbacks.
A first drawback is that the electric portions of each contact unit are connected to the movable part via screws, and this may considerably reduce the overall safety of the switch.
Indeed, should the screws be removed or broken, the movable part of the switch can still pivot relative to the fixed casing about the central pin but cannot cause the electric contacts to also pivot relative to each other.
As a result, in case of failure of the screws, the operator would be allowed to access the area delimited by the barrier, and as the latter is opened or closed there would be no change in the electric signals associated with the safety circuits.
A further drawback is that the construction of this switch is particularly complex, as it requires a great number of parts and relatively long assembly times.
Also, these switches require the use of contact units having a central hole for the passage of the through pin, and for this reason they must be selected from a restricted group of commercially available contacts.
Another significant drawback of the present invention is that any failure of the central pin might cause separation of the movable part from the casing, which would make the switch useless.